1. Technical Field of the Invention
The present invention relates generally to pressure sensing and more particularly to a multi-channel pressure transducer system for cryogenic environments.
2. Discussion of the Related Art
Pressure is one of the most important parameters measured when testing models in wind tunnels. Conventional commercial multi-channel pressure modules have several drawbacks when utilized in models tested in cryogenic environments of wind tunnels such as the National Transonic Facility at NASA Langley Research Center in Hampton, Va. First, it is difficult to maintain the transducer pressure modules within an acceptable operable temperature range above the tunnel temperature of approximately -173.degree. C. The transducers are often placed in incubator packages or supplied with heaters to provide an operable environment. These insulator packages must be relatively thick to provide adequate insulation in a cryogenic environment and accordingly are difficult to install in the relatively small spaces available in test models and aircraft. The heaters also undesirably heat the test airfoil as well at the immediate environment of the transducer, thereby contaminating the boundary layer test parameter.
Also, the application of standard silicon sensors for making pressure measurements in low temperature environments is severely limited in accuracy due to sensor element dopant levels being designed for the commercial market to maximize sensitivity to pressure over a temperature range of -40.degree. C. to 125.degree. C. The main electronic impediment to sensor performance at temperatures below -40.degree. C. is apparently due to the phenomenon of charge carrier freeze-out and temperature dependence of mobility because of the relatively low dopant level of 10.sup.16 boron/cm.sup.3 conventionally used in the piezoresistive Wheatstone bridge elements of the sensor. A proposed solution to charge carrier freeze-out at cryogenic temperatures entails the selection of sensors manufactured with highly doped, e.g., &gt;1.3.times.10.sup.19 boron/cm.sup.3, piezoresistive elements. Sensors fabricated with such high dopant levels are more thermally stable over a wide temperature range extending to low temperatures, but are also somewhat less sensitive to pressure. In addition, such proposed sensors are subject to mechanical failure due to temperature induced stresses and material mismatches at cryogenic temperatures.